A fast nuclear reactor is known in the art having a vessel filled with a liquid-metal coolant contacting an inert gas and which passes through the reactor core and heat exchanger ducts by a coolant circulating means.
In this reactor, the core, the coolant circulating means and heat exchangers are all accommodated in the vessel which is closed by a lid. The reactor vessel is partially filled with a coolant and the space above the level thereof is occupied by an inert gas. The side wall of said reactor vessel is cylindrical in shape and its axis lies in a vertical plane. The cross-sectional shape of the vessel is elliptical.
Other nuclear reactors, similar to the one described above, are known with the cross-sectional shape of the vessel whereof is thereof being circular or oval.
In such reactors, the lid of the vessel is rigidly attached to its side wall, and is made in the form of a flat plate. The lid is also employed to support the coolant circulating means as well as the means for recharging the core and heat exchangers.
In all of the above-mentioned reactors, designing a lid capable of supporting equipment and instrumentation as well as withstanding sharp temperature fluctuations under various operating conditions of the reactor is a difficult engineering problem. As far as robustness is concerned, a flat lid and a vessel having an oval or elliptical cross-sectional shape are the least robust, and cannot preserve their original shape both under various operating conditions of the reactor, and especially in the case of an accidental pressure jump inside the vessel.
The inert gaseous medium, which in the reactor vessel of the above described design occupies the space between the lid and the coolant level, serves as a heat-insulating blanket. The fact that the coolant-inert gas interface is inside the vessel itself may bring about temperature disbalance between those portions of the vessel which are in contact with the coolant and those that are in contact with the inert gas due to different thermal expansions of these portions. In order to eliminate the resulting thermal stresses requires the provision of highly efficient and reliable heat-insulating means at these portions, particularly in the area of attachment of the lid to the vessel.